Bump requests from 2.18.4 to 2.20.0 (#3)

Bumps [requests](https://github.com/requests/requests) from 2.18.4 to 2.20.0.
- [Release notes](https://github.com/requests/requests/releases)
- [Changelog](https://github.com/psf/requests/blob/master/HISTORY.md)
- [Commits](psf/requests@v2.18.4...v2.20.0)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -15,7 +15,7 @@ Pygments==2.2.0
 pyparsing==2.2.0
 pytz==2018.3
 recommonmark==0.4.0
-requests==2.18.4
+requests==2.20.0
 six==1.11.0
 snowballstemmer==1.2.1
 Sphinx==1.7.1